Inbound and Outbound MTAs

When Messaging Server hosts are each supporting many users, and there
is a heavy load of sending SMTP mail, offload the routing task from the Messaging
Server hosts by using separate inbound and outbound MTAs. You can further
share the load by designating different MTAs to handle outgoing and incoming
messages.

Often, both the inbound and outbound MTAs are combined as a single In/Out
SMTP host. To determine if you need one or more MTA hosts, identify the inbound
and outbound message traffic characteristics of the overall architecture.